St. Landry democrat. (Opelousas, La.) 1878-1894, 1887-07-21

Amidst all the marvels which the
world of nature offers to the reflective
and obseryant mind, there are few
which surpass in interest the wonders
revealed by study Of the mechanism
of sight in the animal kingdom. An
exhaustive study of comparative anat-
omy is by no means necessary to real
ize these wonders; in fact, we require
to be reflective as regards obvious and
familiar details, rather than learned in
scientific terms and anatomical discoy-
cries. No very extensive knowledge
of the structure of the eye, for instance,
is necessary to grasp the full signifi-
canoe of the fact that the views obtain
able from the top of ST. Paul's Cathe-
dral or the dizzy pinnacles of the Alps
enter the eye through an aperture of
about an eighth of an inch diameter,
and are reproduced in the interior of
the eye on surface averaging the size
of a sixpence, from which the size.
shape, color, position, and general
peculiarly of every object within range
are accurately conveyed to the brain.
This is only one of the ordinary Won
ders of the human eye, but shares the
fate of many others in that it seldom
occupies a moments thought-no more
thought, in fact, than we bestow upon
the varieties of sight in the lower ami
mails and the lessons we may learn
from them.
Most Of US, in these days of cheap
science manuals, are familiar with the
general structure of our own eyes and
the rudimentary principles of optical
science; and we are SO accustomed to
look upon this particular plan of con-
struction as the only one by which
vision can be accomplished, that it is
impossible not to feel astonished when
observation discloses the extraordinary
variety of structural arrangements
which are to be found in the descend
ing scale of animals. In completely
and minuteness of detail, the eye of
man and the higher animals surpasses
all others; yet its structure may be said
to explain itself, and to be compare
lively simple plan, when regarded in
connection with the laws Of optics. 1t
is exactly such as might be imagined
as consequence of known laws of
light transmission and known proper
ties of matter. A first glimpse, there
fore, into the structure of the eyes of
the lower or invertebrate animals is
somewhat perplexing, owing to the ap-
parent contradictions met with; for the
seemingly simple plan of the mammal
eye is lost in diversity of external
form and internal structure which is
truly marvellous; in some cases, the
elements considered to be essential are
apparently missing; and in others, adj
.ditions are found which have no coun-
terpart in eyes supposed to be more
perfect.
A little reflection will, however, show
that these diversities of structure,
wherever met with, are necessary to
the particular mode of life which their
possessor was designed to lead in the
ranks of creation. In almost all man.
mails for instance, below the Primates,
a third eyelid-or, as it is called, the
'.nictitating" membrane - is found,
,commodiously folded up in the inner
corner of the eyeball, ready at any in
slant to sweep across its surface. This
is found in the eyes of all birds, and Of
such quadruples as require it, its use
being to wash the eye free from any